It's not a specific requirement but it's one more thing that helps. Look at some of the all-time great M/Vs like Flash & Tom Kennedy's Whale, the La Contessa, or the Catbus. If it's something that isn't supposed to have wheels, it's so much more magic if it doesn't appear to have them.
GreyCoyote: "At this rate it wont be long before he is Admiral Fukkit."
Delle: Singularly we may be dysfunctional misfits, but together we're magic.